Southern Oscillation

Southern oscillation refers to a seesaw shift in surface air pressure at
Darwin, Australia and the South Pacific Island of
Tahiti. When the
pressure is high at Darwin it is low at Tahiti and vice versa.
El Nino, and its sister event - La Nina - are the extreme phases of the Southern Oscillation, with El Nino referring to a warming of the eastern tropical Pacific, and La Nina a cooling.

The Southern Oscillation was discovered in 1923 by
Sir Gilbert Thomas Walker a British mathematician and meteorologist. The Southern Oscillation Index (SOI) measures the monthly/seasonal fluctuations in surface air pressure differences at Tahiti and Darwin (Equation = Tahiti - Darwin).